This invention relates to optical add-drop filters and, in particular, to micromechanically active, reconfigurable add-drop filters.
Optical communication systems are beginning to achieve their great potential for the rapid transmission of vast amounts of information. In essence, an optical communication system comprises a source of light, a modulator for impressing information on the light to produce optical signals, an optical fiber transmission line for carrying the optical signals and a receiver for detecting the signals and demodulating the information they carry. Increasingly the optical signals are wavelength division multiplexed signals (WDM signals) comprising a plurality of distinct wavelength signal channels.
Add/drop devices are important components of WDM optical communication systems. Such devices are typically disposed at various intermediate points along the transmission fiber (called nodes) to permit adding or dropping of signal channels at the nodes. Thus, for illustration, an add/drop device would permit a transmission line from New York to Los Angeles to drop off at Chicago signal channels intended for Chicago and to add at Chicago signal channels for New York and Los Angeles. As the number of nodes increases, the number of add/drop devices increases, and their cost and effect on the system become appreciable.
FIG. 1 schematically illustrates a conventional optical add-drop filter 10 known as a microring add-drop filter. The filter 10 comprises, in essence, a pair of optical waveguides 11 and 12 optically coupled by a microscale resonator 13 comprising a waveguide ring closely adjacent each of the wave guides 11, 12. The ring 13 is optically resonant for optical wavelengths xcexi such that nxcexi=C, where C is the circumference of the ring and n is an integer.
In operation, if a set of wavelengths xcex1, xcex2, . . . xcexN is incident on input port I of waveguide 11, any of the wavelengths resonant with the microring resonator will couple across the resonator 13 to waveguide 12 and exit the filter 10 at drop port R. Nonresonant wavelengths will pass the ring structure unperturbed and exit the filter 10 at the through port T. In addition, resonant wavelengths can be added at the add port A and will exit at port T.
The diameter D of the ring is chosen sufficiently small to obtain a desired free spectral range. To obtain a free spectral range of the order of tens of nanometers, D must be less than about 10 micrometers. With such small diameters, the index contrast between the ring and its cladding (the lateral index contrast) must be high to avoid bending losses. Typically, the rings are fabricated with air cladding in the lateral direction.
In view of the high lateral index contrast, the coupling distances d1 and d2 between the ring 13 and waveguides 11, 12, respectfully, must be smallxe2x80x94typically less than 300 nanometers in order to obtain the necessary coupling. In alternative embodiments, the microring resonator 13 can be replaced by a microdisk resonating in whispering gallery modes. Further details concerning the structure and operation of conventional microring and microdisk add-drop filters are set forth in B. E. Little, et al, xe2x80x9cMicroring Resonator Channel Dropping Filtersxe2x80x9d, 15 Journal of Lightwave Technology 998 (1997); B. E. Little, et al., xe2x80x9cUltracompact Sixe2x80x94SiO2 Microring Resonator Optical Channel Dropping Filters, 10 IEEE Photonics Technology Letters 549 (1998); and D. Radfizadeh, et al., xe2x80x9cWave-Guide-Coupled AlGaAs/GaAs Microcavity Ring and Disk Resonators . . . xe2x80x9d, 22 Optics Letters 1244 (1997), each of which is incorporated herein by reference.
While theoretically promising, microring and microdisk add-drop filters are difficult to fabricate with necessary precision. For example, a good quality add-drop filter must essentially eliminate a dropped wavelength so that it does not reach the port T. (The filter must achieve a high extinction ratio for the dropped wavelength.) This elimination requires precise control of the coupling distances d1, d2. But due to their small sizes (less than 300 nm), these distances are difficult to fabricate with the necessary precision. Published results to date have shown only slightly better than 10 dB extinction for the best individual devices.
Another challenge in fabrication is to make microrings or microdisks with precise resonant frequencies. An add-drop filter for telecommunications would need rings or disks with diameters specified and fabricated to better than 1 part in 1500 in order to overlap a dense WDM grid (100 GHz spacing). Moreover, sidewall roughness of the ring adds a further degree of uncertainty to the precise value of the diameter.
Finally it should be noted that the conventional microring and microdisk add-drop filters are fixed in configuration. Once fabricated, the filter will always add and drop the same respective wavelengths. However, in contemplated systems it would be highly advantageous if add-drop filters could be dynamically reconfigured to select and change which wavelength channels are added and dropped.
In accordance with the invention, a tunable, reconfigurable optical add-drop filter comprises a pair of optical waveguides optically coupled by a microring or microdisk resonator wherein the coupling distance between the resonator and at least one of the waveguides is micromechanically controllable. With this arrangement, the degree of coupling can be tuned after fabrication to provide high level extinction of dropped wavelengths and the filter can be dynamically reconfigured. Advantageously, laser radiation is provided to tune the resonant wavelength.